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California politicians shrank our world last month to curb the spread of COVID-19, and it appears to have caused a drop in both greenhouse gases that cause global warming and harmful pollutants in the air we breathe.

When Gov. Gavin Newsom imposed a stay-at-home order in mid-March, businesses responded by permitting many employees to work from home. Freeing them from the need to commute by car each day had an immediate effect: San Diego freeways saw, on average, 23 percent fewer cars during the workweek from the same time a year ago, according to CalTrans. Weekends experienced a 32 percent drop.

On the last Sunday of March, traffic fell 60 percent from the same point a year ago. But the dip in car traffic is also apparent in preliminary data from air-quality sensors placed around the San Diego basin, though the region’s 17 rainy days in March could exaggerate the decline because rain obscures the true amount of pollutants in the air.

Digging Into San Diego’s Air Pollutants

U.S. cities and counties are responsible for meeting air-quality standards set by the Environmental Protection Agency. California cities are also responsible for meeting sometimes more stringent state standards.

But the lack of car travel during the first month of the pandemic response has made those pollutants scarcer in the air we breathe.

Nitrogen dioxide, for instance, is added to the atmosphere when fuel is burned and released from tailpipes. It’s primarily responsible for that brown haze seen on cold mornings across the San Diego skyline. It’s also capable of damaging the lungs and likely the cause of asthma in children, according to the EPA.

Nitrogen dioxide levels decreased by about 33 percent from the year prior around an air-quality sensor adjacent to I-15 near Rancho Bernardo. A sensor just east of I-5 in Chula Vista found a 24 percent drop in nitrogen dioxide levels.

hourly nitrogen dioxide readings rancho carmel drive air quality sensor
hourly nitrogen dioxide readings chula vista air quality sensor

When sunlight hits nitrogen dioxide and other chemicals, it creates ozone – the main ingredient in smog. But when there’s too much ozone – because too many cars are adding it to the atmosphere all at once – it can trap heat and other harmful pollutants, causing toxic breathing conditions.

hourly ozone readings alpine air quality sensor
Hourly ozone readings Kearny Villa Road air quality sensor

The county experienced an overall dip in ozone. Ozone is usually at its worst during the sunny summertime, and it’s one pollutant for which the county has failed to meet both federal and state standards.

A city’s terrain matters a lot when it comes to air quality. Los Angeles rests in a bowl surrounded by mountains, which make it harder for emissions from cars or heavy industries to escape.

San Diego is not situated like Los Angeles, “but we still cook up some ozone a few times a year and therefore we’re out of attainment with air quality standards,” said Bill Brick, chief of the monitoring and technical services division at San Diego Air Pollution Control District.

The county also measures particulate matter (which is essentially fine dust or smoke particles) larger than 2.5 microns, a measurement that stands for one millionth of a meter and too small to be seen with the naked eye.

Particles that small pose the greatest risk to human health because they make their way deep into a person’s lungs and even into the bloodstream, according to the EPA. Cars and trucks rolling along the pavement and wildfires are large sources of such dangerous dust.

Hourly PM readings Lexington Elementary School Air quality sensor
hourly PM 2.5 reading donovan state prison air quality sensor

Those particles dropped almost 19 percent during the stay-at-home order, as measured by a sensor at Lexington Elementary School in El Cajon. Similarly, a sensor in Otay Mesa near the U.S.-Mexico border experienced an almost 40 percent drop.

Penelope Quintana, an environmental health expert at San Diego State University, studies the health effects of fine dust pollution from traffic near the U.S.-Mexico border.

“If there’s something good to come out of this (COVID-19) slowdown, it’s showing how our behavior can reduce pollution,” Quintana said. “Maybe we can take some lessons from that of what’s possible.”

The Sudden Rise of Telework

In human history, many previously incurable maladies preceded the COVID-19 pandemic. In all of Earth’s history, our burning of fossil fuels is unprecedented. Humans didn’t begin doing that en masse until the Industrial Revolution.

It’s difficult to say what the halt in the global economy means for slowing human-caused climate change.

Measurements of global carbon dioxide, the most persistent greenhouse gas (which are also emitted from tailpipes) taken at Mauna Loa Observatory in Hawaii by San Diego’s Scripps Institution of Oceanography continue to tick up. On March 11, geophysicist Ralph Keeling said fossil fuel use would have to drop 10 percent around the world for an entire year to affect the steadily creeping curve.


It’s also hard to say what this jolt to San Diego driving habits means for local greenhouse gas reduction goals.

Hasan Ikhrata, executive director of the regional planning agency SANDAG, is hopeful these telework habits will stick.

The agency expects to publish a vision of its 2021 transportation plan soon. Agencies typically propose pollution-cutting measures like building bike lanes, adding sidewalks and expanding public transit to get people out of their cars.

“The best way is to eliminate the trip altogether,” Ikhrata told Voice of San Diego.

During a March 27 address to SANDAG’s board of directors, Ikhrata said he hoped many employers will continue to permit employees to work from home after COVID-19.

“That’s definitely going to make it easier to meet some of our requirements and will be good for our system,” he told the board.

Ikhrata said SANDAG should put more resources into its iCommute program, created in 2014 to increase commuting by transit, biking, walking and telework.

The agency got a federal grant to conduct a telework study in 2016 with four San Diego companies, Ikhrata said. By the end, 60 percent of the managers said they would allow additional employees to work from home.

But the data from that study wasn’t rich enough, Ikhrata said, because it was difficult to get businesses to participate. He thinks the COVID-19 pandemic experience will encourage companies to join in on a second pilot study.

By 2035, SANDAG needs the region to cut greenhouse gas emissions 19 percent from 2005 levels to meet a draft target set by the California Air Resources Board.

“Maybe (telework) will be the most cost-effective way to do it,” Ikhrata said.

Telework isn’t part of the city of San Diego’s Climate Action Plan, approved in 2015, which called for half of people living close to high-frequency transit to bike, walk or take transit to work by 2035. But telework could become part of the updated plan due in 2021, said Ashley Rosia-Tremonti, the city’s sustainability manager.

Transportation accounts for 55 percent of the total greenhouse gases emitted and tracked by the city of San Diego.

“There will be a lot of data coming out of this time period to help us make those decisions down the road,” Rosia-Tremonti said.

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